Optical absorbers for use with solar cells are more economically attractive if they exhibit high efficiency and can be made from thin films. Absorbers based on various combinations of at least copper, indium, gallium, and selenium (CuInxGa1−xSe2 or “CIGS”) have been widely studied to meet these performance goals. CIGS has a strong absorption coefficient for visible light making it possible to use thinner absorber layers further reducing costs of assembled solar cells.
This composition is often described as having one Cu atom for every In and/or Ga atom (i.e., Cu/(In+Ga)=1, where the atomic symbols refer here to the number of each type of atom). However, high-efficiency absorbers are Cu-poor (Cu/(In+Ga)<1). For example, Weber et al. describe the preparation of a solar cell using a metallic precursor layer deposited by sputtering from a Cu85Ga15 target and an In target to produce a selenized absorber having a Cu/(In+Ga) ratio of 0.8 (Weber, A. et al. 2011 “Fast Cu(In, Ga)Se2 formation by processing Cu—In—Ga precursors in selenium atmosphere” 37th IEEE Photovoltaic Specialists Conference, Seattle, Wash.; Jun. 19, 2011). Absorbers are typically also Ga-poor, having a Ga/(In+Ga) ratio<0.4.
However, Cu(In,Ga) metal precursor films (used to form CIGSe via chalcogenization) with these preferred atomic ratios are multi-phasic and tend to separate into discrete domains when deposited, especially when exposed to processing temperatures above about 155° C. This phase inhomogeneity can be observed in X-Ray diffraction and also in various microscopy techniques, such as optical microscopy, scanning electron microscopy, and atomic force microscopy (the roughness tends to go hand-in-hand with the multi-phasic nature of the film). For example, Weber et al. describe that at room temperature, the metal precursor layer contains the crystalline phases In and at least one Cux(In,Ga)y phase (though not clearly assigned), and that upon heating, an In melt is formed, with a resultant decreasing In/Ga ratio. This phase separation makes it difficult to form laterally uniform compositions, and after selenization, the resulting CIGS absorbers are also non-uniform, reducing the achievable open-circuit voltage and fill factor, and therefore, the overall performance (efficiency).
Attempts have been made to fabricate laterally uniform CIGS layers by physical vapor deposition (PVD) of the metals (followed by chalcogenization) by sequentially using sputtering targets such as In with alloyed sputtering targets such as Cu0.75—Ga0.25, Cu0.60—Ga0.40, or Cu0.85—Ga0.15 to force uniform deposition of layers having the desired composition. However, upon deposition or upon subsequent heating, the film always comes out laterally non-uniform, as indicated by optical and electron microscopy.
U.S. Patent Application Nos. 2012/0313200 to Jackrel and 2010/0248219 to Woodruff describe that even if a uniform precursor layer is deposited, the materials may flow and migrate during processing to result in a different layer uniformity, roughness, homogeneity and quality and number of crystals within the layer. These publications describe the use of particles mixed with a carrier liquid to form an ink and the ink is used to coat a substrate to form a precursor layer.
Further performance improvement in CIGS absorbers can be achieved by grading the bandgap across the thickness of the CIGS layer. As outlined above, it is challenging to control the lateral uniformity when starting from today's solutions for Cu(In,Ga) sputtering followed by chalcogenization. Controlling both lateral uniformity and compositional depth grading for CIGSe or CIGSSe, therefore, has proven even more challenging. Other CIGS growth methods try to improve the control over both lateral uniformity and compositional depth grading. For example, grading can be achieved by varying the In/Ga ratio through the thickness of the CIGSe film. This is most often done by co-evaporation which has proven to be challenging on the manufacturing floor. Similarly, the preparation of graded absorbers can be performed using chalcogenide targets. This, however, results in a decreased sputtering rate compared to metal deposition, decreasing throughput, and increasing capital expenditure, in addition to an increased cost in sputter target manufacturing compared to metal targets.